7. Thermodynamics

Question 1

Know to find solid, liquid, gas, sublimation/deposition, melting/freezing, condensation/vaporization in phase diagram. What’s the triple point vs critical point?

Answer 1

phase change on lines, states of matter on spaces (left to right: solid, liq, gas)
all phases simul vs no distinction between phases: liquid evap at super fast rate –> density of liq and gas = same => supercritical fluid
temp on x axis and pressure on y axis

Question 2

Know how to read a heating curve

Answer 2

phases on slants, phase changes on flat lines (all phase changes = isothermal ie. no temp change at all), y-axis = temp, x-axis = heat added

Question 3

What are the 0th vs 1st vs 2nd laws of thermodynamics?

Answer 3

transitive property with thermal equil; heat transfers from hot to cold vs energy cannot be created nor destroyed vs entropy inc spontaneously

Question 4

High vapor pressure means…

Answer 4

It REALLY wants to evaporate (ex: CO2; Fe has low vapor pressure)
usually lighter molec, high KE

Question 5

How does inc/dec temp affect exothermic rxns?

Answer 5

Inc temp shifts to left, dec temp shifts to right

Question 6

How do you know if there’s more entropy in a rxn?

Answer 6

Look at phase changes or moles; look to see if products are MORE ORDERLY (ex: dimer formation = dec in entropy)

Question 7

What does system do w/ heat for endothermic vs exothermic?

Answer 7

system absorbs heat vs system releases heat

Question 8

Isothermal vs adiabatic vs isobaric vs isovolumetric/isochoric

Answer 8

Temp of system is constant (looks like a shallow curve on P-V graph) vs no heat exchange occurs (looks hyperbolic or deeper curve on a P-V graph) vs pressure of system is constant vs volume of system is constant

Question 9

Bond enthalpy formula

Answer 9

DeltaH0rxn = sum(deltaH) for bonds broken minus sum(deltaH) for bonds formed

Question 10

Bond dissociation energy aka bond enthalpy

Answer 10

THE ENERGY IT TAKES TO BREAK SPECIFC BONDS IN A GAS PHASE; NEEDING ENERGY TO BREAK A BOND IS ENDOTHERMIC. BDE ITSELF = ALWAYS A POS VALUE! (So don’t confuse BDE with DeltaH0rxn b/c DeltaH0rxn could be neg if you calculate it)
This means forming a bond releases energy —> exothermic

Question 11

Entropy of univ formula

Answer 11

DeltaS for universe = deltaS for system + deltaS for surroundings > 0

Entropy of system can dec if entropy of surroundings inc by same amount. In an isolated system, entropy always inc

Question 12

If deltaG = 0, system in in equil. What does it mean for deltaH and TdeltaS?

Answer 12

DeltaH = TdeltaS

Question 13

Relationship with deltaU = Q - W. And know the relations in adiabatic, isothermal and isovolumetric processes

Answer 13

pos deltaU = inc temp; neg deltaU = dec temp
pos Q = heat flows into system; neg Q = heat flows out of system
pos W = work is done by system => expansion; neg W = work is done on system => compression

Question 14

State functions vs process functions

Answer 14

Describes a system in equil state (ex: pressure, density, temp, vol, heat/enthalpy, internal energy, Gibbs free energy, entropy) vs describes how a systems reached equil quantitatively (ex: Q and W)

Question 15

Standard conditions vs STP

Answer 15

1 M, 1 atm, 25°C aka 298 K vs 0°C aka 273 K, 1 atm

Question 16

What’s enthalpy?

Answer 16

deltaH, pos = endothermic and neg = exothermic. Same thing as Q but under constant pressure

Question 17

What’s the specific heat capacity? What’s its value for water?

Answer 17

1 cal/gK or 4.184 J/gK

Amount of heat required to raise temp of one mass unit by 1 degree C or 1 K. INVERSE TO DELTAT

Question 18

Examples of constant-pressure calorimeter vs constant-volume calorimeter

Answer 18

Coffee cup vs bomb calorimeter/decomposition vessel

Calorimeter is suppose to be an isolated system but when it’s not sealed correctly, it’ll be an open system

Question 19

How to find heat energy during a phase change?

Answer 19

q = mL

L - in cal/g; heat of transformation or latent heat of substance; represents enthalpy of isothermal process

Question 20

Enthalpy of fusion vs vaporization

Answer 20

Enthalpy at solid/liquid change vs liq/gas change

Question 21

Standard heat of formation formula

Answer 21

DeltaH0 = sumn(deltaHf0) of products - sumn(deltaHf0) of reactants

n = mol
DeltaHf0 = standard enthalpy of formation (amount of heat lost or gained in a rxn); usually given, but know that standard enthalpy of formation for any element in standard state = zero

Question 22

Entropy formula of a reversible process

Answer 22

DeltaS = Qrev/T

DeltaS=change in entropy
Qrev=heat that’s gained or lost in a reversible process
T=temp in K

Question 23

DeltaG for anabolic vs catabolic rxns

Answer 23

> 0 vs < 0

Question 24

Hess’ Law

Answer 24

Enthalpy change = same for a rxn consisting of a single step or multiple steps. 2 ways to manipulate to match the rxn you’re looking for: flip rxn (you also have to flip sign of given deltaH) or multiply by constant (you also have to multiply that same constant to given deltaH)

Question 25

Most NEG change in enthalpy and entropy means?

Answer 25

Least change

Question 26

Why is temp necessary for phase change?

Answer 26

To overcome intermolecular forces

Question 27

Kinetic vs thermodynamic product

Answer 27

Lower Ea needed —> forms faster, get it at low temps, ends with higher deltaG compared to thermodynamic product —> less stable product; IRREVERSIBLE vs high Ea needed —> forms slower, get it at high temps, ends with lower deltaG than kinetic product —> more stable product; REVERSIBLE

Question 28

The most neg rate of combustion means…?

Answer 28

Reactants have more energy than products —> less stable

Question 29

What kind of rxn vessel is best to do rxns?

Answer 29

low thermal conductivity (we want rxn vessel to be insulating), low thermal expansion (we don’t want rxn vessel to expand and break under high temps), high melting pt (we want rxn vessel to w/stand lots of heat if a rxn = exothermic)